A typical gas turbine engine includes a compressor section, a combustor and a turbine section. Working fluid flowing through the gas turbine engine is compressed in the compressor section to add energy to the working fluid. The compressed working fluid exits the compressor section and enters the combustor. In the combustor, the working fluid is mixed with a supply of fuel and ignited. The products of combustion are then flowed through the turbine section where energy is extracted from the working fluid. A portion of the extracted energy is transferred back to the compressor section to compress incoming working fluid and the remainder may be used for other functions.
Gas turbine engines are required to function efficiently over a range of operating conditions. For a gas turbine engine used in aircraft applications, low power operation corresponds to idle, high power operation corresponds to take-off and climb, with cruise and approach/descent falling in an intermediate thrust region between low and high power. At low power, fuel/air ratios must be kept relatively rich to avoid blow-out. Blow-out occurs when the fuel/air ratio within the combustor drops below a lean stability limit. As a result of the low combustion temperature and pressure, combustion efficiency is relatively low. At high power, the fuel/air ratio is near stoichiometric to maximize efficiency.
The combustion process generates numerous byproducts such as smoke particulate, unburned hydrocarbons, carbon monoxide, and oxides of nitrogen. At low power, the lower combustion efficiency results in the production of unburned hydrocarbons and carbon monoxide. At high power, the production of oxides of nitrogen increases as the operating temperature and residence time increase. Residence time is defined as the amount of time the combustion mixture remain above a particular temperature. Reducing the operating temperature may reduce the output of the gas turbine engine. Reducing the residence time, may result in less efficient combustion and higher production of carbon monoxide. For environmental reasons, these byproducts are undesirable. In recent years, much of the research and development related to gas turbine engine combustion has focused on reducing the emission of such byproducts.
A significant development in gas turbine engine combustors has been the introduction of multiple stage combustors. A multiple stage combustor typically includes a pilot stage, a main stage, and possibly one or more intermediate stages. An example of such a combustor is disclosed in U.S. Pat. No. 4,265,615, issued to Lohmann et al and entitled "Fuel Injection System for Low Emission Burners".
At low power only the pilot stage is operated. This permits fuel/air ratios nearer to stoichiometric and the efficiency at idle is thereby increased and the production at idle of unburned hydrocarbons and carbon monoxide is reduced. At high power the pilot stage and one or more of the other stages is operated. Having multiple stages reduces the residence time within each particular stage, relative to a single large combustion chamber. The lower residence time results in lower production of oxides of nitrogen. Having multiple stages also permits the equivalence ratio to be optimized over a range of operating conditions. As a result of having multiple stages rather than a single stage, the emission of unwanted combustion byproducts is reduced and the overall efficiency is improved.
A fuel supply system for a staged combustor is required to supply fuel to each stage as needed and to evenly distribute the fuel between the fuel injectors in each stage. One way of accomplishing this is to have an annular manifold surrounding each stage and a switching valve which distributes the fuel to each of the manifolds. An example of such a fuel supply system is described in U.S. Pat. No. 4,903,478 issued to Seto et al and entitled "Dual Manifold Fuel System". As described therein, the fuel system includes a first fuel manifold, a second fuel manifold, a fuel control directing fuel to the manifolds, and a shut-off valve disposed between the fuel control and the second fuel manifold. The shut-off valve opens and closes in response to the operating condition of the engine. A drawback to this configuration is that with the shut-off valve closed, fuel may drain from the main fuel manifold. Before starting the main stage, the fuel manifold and associated fuel lines have to be pre-filled. Pre-filling the main manifold and associated fuel lines may degrade the responsiveness of the fuel supply system unless this limitation is accommodated in some fashion.
In U.S. Pat. No. 4,964,270, issued to Taylor et al and entitled "Gas Turbine Engine Fuel System", another type of dual manifold system is disclosed. In the system described therein, a solenoid valve prevents fuel flow through a starter manifold during normal engine operation. The fuel system further includes a drain system to purge the starter manifold of fuel. The fuel is purged, rather than permitting small amounts of fuel to flow through the manifold for cooling, to prevent coking of fuel in the fuel injectors connected to the starter manifold.
Another type of fuel supply system utilizes a flow divider valve, or fuel distribution valve, for each stage. The flow divider valve distributes the flow to the fuel injectors in each stage. An example of a flow divider valve is described in U.S. Pat. No. 5,003,771 issued to Kester et al and entitled "Fuel Distribution Valve for a Combustion Chamber". In this type of system, the function of the shut-off valve described above is incorporated into the flow divider valve. The flow divider valves replace the large manifolds and, as a result, reduces the amount of pre-filling necessary.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop responsive and robust fuel supply systems for staged combustors.